Crystalline sodium silicates having a sheet structure are used as water softeners and builders in detergent and cleaning compositions. Crystalline sodium silicates having a sheet structure, in particular those having an SiO.sub.2 :Na.sub.2 O molar ratio of (1.9 to 2.1):1, are suitable for this purpose. These are also referred to as SKS grades. Depending on the preparation and crystallization conditions, there are several modifications of this sodium silicate, referred to as .alpha., .beta., .gamma. and .delta. phases. For the above applications, the .delta. modification (SKS-6) is of particular interest.
The prior art already discloses a series of processes which can be used to prepare crystalline sodium silicates having a sheet structure.
EP 0 293 640 B1 describes a process for the preparation of crystalline sodium silicates having a sheet structure and an SiO.sub.2 :Na.sub.2 O molar ratio of 1.9:1 to 3.5:1 from a water glass solution having a solids content of from 20 to 65% by weight, which comprises treating the water glass solutions in a spray-drying zone to form a pulverulent amorphous sodium silicate having a maximum ignition loss of 20% by weight, the waste gas leaving the spray-drying zone having a temperature of at least 140.degree. C., and then heat-treating the spray-dried sodium silicate in an ignition zone containing an agitated fixed bed at temperatures of from 500 to 800.degree. C. for from 1 to 60 minutes in the presence of at least 10% by weight of a recovered product which has been obtained by mechanical comminution of crystalline sodium silicate discharged from the ignition zone. It is a disadvantage of this process that to ensure trouble-free operation, a sufficiently large amount of the recovered product must be re-used. As a result, the process becomes complex.
EP 0 425 428 B1 likewise describes a process for the preparation of crystalline sodium silicates having a sheet structure and the aforementioned molar ratio and also a water content of less than 0.3% by weight from a water glass solution having at least 20% by weight of solids, which comprises obtaining the water glass solution by reacting quartz sand with sodium hydroxide solution, treating the water glass solution in a spray-drying zone at 200 to 300.degree. C. and for a residence time of from 10 to 25 seconds, the temperature of the waste gas leaving the spray-drying zone being from 90 to 130.degree. C., to form a pulverulent sodium silicate, which is then introduced into an inclined rotary kiln fitted with a solids agitation device and treated countercurrently with flue gas at temperatures of from 500 to 850.degree. C. for a period of up to 60 minutes to form crystalline sodium silicate.
The rotary kiln is insulated in such a way that the temperature of its outer wall is less than 60.degree. C. The crystalline sodium silicate leaving the rotary kiln is comminuted to particle sizes of from 0.1 to 12 mm using a mechanical crusher. The sodium silicate obtained by this process is mainly present in the .delta. modification.
A process for the preparation of crystalline sodium silicates having a sheet structure from sand and soda is described in EP-A-0 436 835. This involves melting sand and soda in an SiO.sub.2 :Na.sub.2 O molar ratio of 2 to 3.5 at temperatures of from 1200 to 1400.degree. C., cooling the melt, grinding the lumpy water glass formed to particle sizes of less than 2 mm and then treating the granules at from 600 to 800.degree. C. in an elongate reaction zone with mechanical circulation for from 10 to 120 minutes and subsequently grinding them to a particle fineness of less than 1 mm. This process mainly produces .alpha.-Na.sub.2 Si.sub.2 O.sub.5.
EP-A-0 502 325 describes a process similar to that in EP-A-0 425 428, except that a spray-dried pulverulent amorphous sodium silicate is ground before being introduced into the rotary kiln. In this process the waste gas leaving the rotary kiln has only a low dust content and the rotary kiln can be filled to a greater degree.
A further process variant, described in EP-A-0 548 509, is the process already described in EP-A-0 425 428, the difference being that the spray-dried pulverulent amorphous sodium disilicate is ground and, after grinding, is introduced into a rotary kiln, which is fitted with a solids agitation device and is heated externally via the wall and has several different internal temperature zones, and treated therein at temperatures of from 400 to 800.degree. C. for from 1 to 60 minutes to form crystalline sodium disilicate. The product obtained is mainly the .delta. modification of Na.sub.2 Si.sub.2 O.sub.5.
A disadvantage of all the above processes is that it is very difficult to prepare, under reproducible conditions, Na.sub.2 Si.sub.2 O.sub.5 which consists of virtually 100% of the .delta. modification. Even a series of involved process steps is unable to achieve this aim completely.
The object of the present invention is, therefore, to provide a simple and economical process for reproducibly preparing crystalline sodium silicates having a sheet structure and an SiO.sub.2 :Na.sub.2 O molar ratio of (1.9 to 2.1):1. The sheet silicate should have as high a .delta. phase content as possible.